Although tremendous advances have been made in elucidating the genomic abnormalities that cause malignant cancer cells, currently available chemotherapy remains unsatisfactory, and the prognosis for the majority of patients diagnosed with cancer remains dismal.
Heat shock proteins (HSPs) are a class of chaperone proteins that are up-regulated in response to elevated temperature and other environmental stresses, such as ultraviolet light, nutrient deprivation, and oxygen deprivation. HSPs act as chaperones to other cellular proteins (called client proteins) and facilitate their proper folding and repair, and aid in the refolding of misfolded client proteins. There are several known families of HSPs, each having its own set of client proteins. The Hsp90 family is one of the most abundant HSP families, accounting for about 1-2% of proteins in a cell that is not under stress and increasing to about 4-6% in a cell under stress. Inhibition of Hsp90 results in degradation of its client proteins via the ubiquitin proteasome pathway. Unlike other chaperone proteins, the client proteins of Hsp90 are mostly protein kinases or transcription factors involved in signal transduction, and a number of its client proteins have been shown to be involved in the progression of cancer.
Protein kinases (PKs) play a role in signal transduction pathways regulating a number of cellular functions, such as cell growth, differentiation, and cell death. PKs are enzymes that catalyze the phosphorylation of hydroxy groups on tyrosine, serine and threonine residues of proteins. There are two classes of PKs: protein tyrosine kinases (PTKs), which catalyze the phosphorylation of tyrosine kinase residues, and the serine-threonine kinases (STKs), which catalyze the phosphorylation of serine or threonine residues. Growth factor receptors with PTK activity are known as receptor tyrosine kinases. Receptor tyrosine kinases are a family of tightly regulated enzymes, and the aberrant activation of various members of the family is one of the hallmarks of cancer. The receptor tyrosine kinase family can be divided into subgroups that have similar structural organization and sequence similarity within the kinase domain. The members of the type III group of receptor tyrosine kinases include platelet-derived growth factor (PDGF) receptors (PDGF receptors alpha and beta), colony-stimulating factor (CSF-1) receptor (CSF-1R, c-Fms), Fms-like tyrosine kinase (FLT3), and stem cell factor receptor (c-kit). FLT3 is primarily expressed on immature hematopoietic progenitors and regulates their proliferation and survival.
Hematologic cancers, also known as hematologic or hematopoietic malignancies, are cancers of the blood or bone marrow; including leukemia and lymphoma. Acute myelogenous leukemia (AML) is a clonal hematopoietic stem cell leukemia that represents about 90% of all acute leukemias in adults with an incidence of 3.9 per 100,000 (See e.g., Lowenberg et al., N. Eng. J. Med. 341: 1051-62 (1999) and Lopesde Menezes, et al, Clin. Cancer Res. (2005), 11(14):5281-5291). While chemotherapy can result in complete remissions, the long term disease-free survival rate for AML is about 14% with about 7,400 deaths from AML each year in the United States. Approximately 70% of AML blasts express wild type FLT3 and about 25% to about 35% express FLT3 kinase receptor mutations which result in constitutively active FLT3. Two types of activating mutations have been identified in AML patients: internal tandem duplications (ITDs) and point mutation in the activating loop of the kinase domain. FLT3-ITD mutations in AML patients is indicative of a poor prognosis for survival, and in patients who are in remission, FLT3-ITD mutations are the most significant factor adversely affecting relapse rate with 64% of patients having the mutation relapsing within 5 years (see Current Pharmaceutical Design (2005), 11:3449-3457). The prognostic significance of FLT3 mutations in clinical studies suggests that FLT3 plays a driving role in AML and may be necessary for the development and maintenance of the disease.
Mixed Lineage Leukemia (MLL) involve translocations of chromosome 11 band q23 (11q23) and occur in approximately 80% of infant hematological malignancies and 10% of adult acute leukemias. Although certain 11q23 translocation have been shown to be essential to immortalization of hematopoietic progenitors in vitro, a secondary genotoxic event is required to develop leukemia. There is a strong concordance between FLT3 and MLL fusion gene expression, and the most consistently overexpressed gene in MLL is FLT3. Moreover, it has been shown that activated FLT3 together with MLL fusion gene expression induces acute leukemia with a short latency period (see Ono, et al., J. of Clinical Investigation (2005), 115:919-929). Therefore, it is believed that FLT3 signally is involved in the development and maintenance of MLL (see Armstrong, et al., Cancer Cell (2003), 3:173-183).
The FLT3-ITD mutation is also present in about 3% of cases of adult myelodysplastic syndrome and some cases of acute lymphocytic leukemia (ALL) (Current Pharmaceutical Design (2005), 11:3449-3457).
FLT3 has been shown to be a client protein of Hsp90, and 17AAG, a benzoquinone ansamycin antibiotic that inhibits the activity of Hsp90, has been shown to disrupt the association of Flt3 with Hsp90. The growth of leukemia cell that express either wild type FLT3 or FLT3-ITD mutations was found to be inhibited by treatment with 17AAG (Yao, et al., Clinical Cancer Research (2003), 9:4483-4493).
c-Kit is a membrane type III receptor protein tyrosine kinase which binds Stem Cell Factor (SCF) to its extracellular domain. c-Kit has tyrosine kinase activity and is required for normal hematopoiesis. However, mutations in c-kit can result in ligand-independent tyrosine kinase activity, autophosphorylation, and uncontrolled cell proliferation. Aberrant expression and/or activation of c-Kit has been implicated in a variety of pathologic states. For example, evidence for a contribution of c-Kit to neoplastic pathology includes its association with leukemias and mast cell tumors, small cell lung cancer, testicular cancer, and some cancers of the gastrointestinal tract (such as GIST) and central nervous system. In addition, c-Kit has been implicated in carcinogenesis of the female genital tract sarcomas of neuroectodermal origin and in Schwann cell neoplasia associated with neurofibromatosis. (Yang et al., J Clin Invest. (2003), 112:1851-1861; Viskochil, J Clin Invest. (2003), 112:1791-1793).
c-Kit has been shown to be a client protein of Hsp90, and Hsp90 inhibitor 17AAG, a benzoquinon ansamycin, has been shown to induce apoptosis in Kasumi-1 cells, an acute myeloid leukemia cell line that harbors a mutation in c-kit. In addition, benzoquinone ansamycins have shown evidence of therapeutic activity in clinical trials for a number of cancers.
Epidermal Growth Factor Receptor (EGFR) is a member of the type I tyrosine kinase family of growth factor receptors which play critical roles in cellular growth, differentiation, and survival. Activation of these receptors typically occurs via specific ligand binding which results in hetero- or homodimerization between receptor family members, with subsequent autophosphorylation of the tyrosine kinase domain. Specific ligands which bind to EGFR include epidermal growth factor (EGF), transforming growth factor α (TGFα), amphiregulin and some viral growth factors. Activation of EGFR triggers a cascade of intracellular signaling pathways involved in both cellular proliferation (the ras/raf/MAP kinase pathway) and survival (the PI3 kinase/Akt pathway). Members of this family, including EGFR and HER2, have been directly implicated in cellular transformation.
A number of human malignancies are associated with aberrant or overexpression of EGFR and/or overexpression of its specific ligands (Gullick, Br. Med. Bull. (1991), 47:87-98; Modijtahedi and Dean, Int. J. Oncol. (1994), 4:277-96; Salomon, et al., Crit. Rev. Oncol. Hematol. (1995); 19:183-232). Aberrant or overexpression of EGFR has been associated with an adverse prognosis in a number of human cancers, including head and neck, breast, colon, prostate, lung (e.g., NSCLC, adenocarcinoma and squamous lung cancer), ovaries, gastrointestinal cancers (gastric, colon, pancreatic), renal cell cancer, bladder cancer, glioma, gynecological carcinomas, and prostate cancer. In some instances, overexpression of tumor EGFR has been correlated with both chemoresistance and a poor prognosis (Lei, et al., Anticancer Res. (1999), 19:221-8; Veale, et al., Br. J. Cancer (1993); 68:162-5).
Gefitinib, a chemotherapeutic agent that inhibits the activity of EGFR, has been found to be highly efficacious in a subset of lung cancer patients that have mutations in the tyrosine kinase domain of EGFR. In the presence of EGF, these mutants displayed two to three times higher activity than wild type EGFR. In addition, wild type EGFR was internalized by the cells and down-regulated after 15 minutes, where as mutant EGFR was internalized more slowly and continued to be activated for up to three hours (Lynch, et al., The New England Journal of Medicine (2006), 350:2129-2139).
Gliomas are another type of cancer that is characterized by amplification and/or mutation of the EGFR gene. One of the most common mutations in the EGFR gene is a deletion of exons 2-7 which results in a truncated form of EGFR in which amino acids 6-273 of the extracellular domain are replaced with a single glycine residue. This mutation is called EGFRvIII and is expressed in about half of all glioblastomas. EGFRvIII is unable to bind EGF and TGFα and has constitutive, ligand-independent tyrosine kinase activity. Hsp90 co-purifies with EGFRvIII indicating that Hsp90 complexes with EGFRvIII. Moreover, Hsp90 inhibitor geldanamycin, a benzoquinone ansamycin antibiotic, was able to decrease the expression of EGFRvIII indicating that interaction with Hsp90 is essential to maintain high expression levels of EGFRvIII (Lavictoire, et al., Journal of Biological Chemistry (2003), 278(7):5292-5299).
The Raf family of proto-oncogenes (A-raf, B-raf and C-raf) was first identified when C-raf was discovered due to its homology with v-raf, the transforming gene of the mouse sarcoma virus 3611. A-raf was later discovered by screening a cDNA library under low stringency conditions using a v-raf probe, and B-raf was discovered due to its homology with C-Rmil, a transforming gene in avaian retrovirus Mill Hill No. 2. The Raf family of proteins is involved in the Ras/Raf/MEK/ERK pathway, referred to herein as the “MAP kinase pathway” (MEK stands for “MAPK/ERK kinase” and ERK stands for “extracellularly regulated kinases”), which has been implicated in the genesis and progression of many human cancers through upregulation of cell division and proliferation. All raf proteins are serine/theronine kinases which are capable of activating the MAP kinase pathway. However, B-raf is far more potent at activating this pathway than A-raf or C-raf, and mutations in the gene encoding B-raf are more common in cancer. For example, B-raf mutations have been identified in 60% to 70% of malignant melanomas, 83% of anaplastic thyroid carcinoma, 35% to 69% of papillary thyroid carcinoma, 4% to 16% of colon cancer, 63% of low-grade ovarian carcinoma, 15% of Barrett's esophageal carcinoma, 4% of acute myeloid leukemia, 3-4.8% of head and neck squamous cell carcinoma, 2%-3% of non-small-cell lung cancer, 2% of gastric carcinoma, 2% of non-Hodgkins lymphoma and has been reported in glioma, saroma, breast cancer, cholangiocarcinoma, and liver cancer. Most mutations in B-raf that have been found in human cancers are point mutations that occur in the kinase domain and are clustered in exons 11 and 15 of the gene which contains several regulatory phosphorylation sites (S446, 5447, D448, D449, T599, and S602). (Beeram, et al., Journal of Clinical Oncology (2005), 23(27):6771-6790). The most prevalent mutation is the T1799A transversion mutation which accounts for more than 80% of mutations in the BRAF gene and results in a V600E mutation in B-raf. The V600E was formerly designated V599E (the gene mutation was designated T1796A) due to a mistake in the GenBank nucleotide sequence NM 004333. The corrected GenBank sequence is NT 007914 and designates the protein mutation as V600E and the gene mutation as T1799A. This corrected numbering will be used herein. This mutation is thought to mimic phosphorylation in the activation segment of B-raf since it inserts a negatively charged residue near two activating phosphorylation sites, T599 and 5602, and thus results in constitutively active B-raf in a Ras independent manner. (Xing, M., Endocrine-Related Cancer (2005), 12:245-262).
Treatment of cancer cells with 17AAG has been shown to stimulate the degradation of B-raf, and mutant forms of B-raf have been shown to be more sensitive to degradation than the wild type. For example, when melanoma cell line A375 which contain the V600E mutation was treated with 17AAG, B-raf was degraded more rapidly than in CHL cells which contained wild type B-raf. Other B-raf mutants (e.g., V600D, G469A, G469E, G596R, G466V, and G594V) were a found to be degraded more rapidly than wild type B-raf when transvected into COS cells. However, B-raf mutants E586K and L597V were not sensitive to degradation when cells were treated with 17AAG. Therefore, it is believed that wild type B-raf in its activated form is a client protein of Hsp90 and that most mutated forms of B-raf are more dependent on Hsp90 for folding, stability and/or function than the wild type protein. (Dias, et al., Cancer Res. (2005), 65(23): 10686-10691).
BCR-ABL is an oncoprotein with tyrosine kinase activity and has been associated with chronic myelogenous leukemia (CML), with a subset of patients with acute lymphocytic leukemia (ALL) and with a subset of patients with acute myelogenous leukemia (AML). In fact, the BCR-ABL oncogene has been found in at least 90-95% of patients with CML, 20% of adults with ALL, 5% of children with ALL, and in about 2% of adults with AML. The BCR-ABL oncoprotein is generated by the translocation of gene sequences from the c-ABL protein tyrosine kinase on chromosome 9 into the BCR sequences on chromosome 22, producing the Philadelphia chromosome. The BCR-ABL gene has been shown to produce at least three alternative chimeric proteins, p230 Bcr-Abl, p210 Bcr-Abl, and p190 Bcr-Abl which have unregulated tyrosine kinase activity. The p210 Bcr-Abl fusion protein is most often associated with CML, while the p190 Bcr-Abl fusion protein is most often associated with ALL. Bcr-Abl has also been associated with a variety of additional hematological malignancies including granulocytic hyperplasia, myelomonocytic leukemia, lymphomas and erythroid leukemia.
Studies have shown that lowering the expression or activity of Bcr-Abl is effective in treating Bcr-Abl-positive leukemias. For example, agents such as As2O3 which lower Bcr-Abl expression have been shown to be highly effective against Bcr-Abl leukemias. In addition, inhibition of Bcr-Abl tyrosine kinase activity by Imatinib (also known as STI571 and Gleevec) induces differentiation and apoptosis and causes eradication of Bcr-Abl positive leukemia cells both in vivo and in vitro. In patients with CML in the chronic phase, as well as in a blast crisis, treatment with Imatinib typically will induce remission. However, in many cases, particularly in those patients who were in a blast crisis before remission, the remission is not durable because the Bcr-Abl fusion protein develops mutations that cause it to be resistance to Imatinib. (See Nimmanapalli, et al., Cancer Research (2001), 61:1799-1804; and Gone, et al., Blood (2002), 100:3041-3044).
Bcr-Abl fusion proteins exist as complexes with Hsp90 and are rapidly degraded when the action of Hsp90 is inhibited. It has been shown that geldanamycin, a benzoquinone ansamycin antibiotic that disrupts the association of Bcr-Abl with Hsp90, results in proteasomal degradation of Bcr-Abl and induces apoptosis in Bcr-Abl leukemia cells.
NPM-ALK is another fusion protein that has been associated with the genesis and progression of certain types of cancers such as anaplastic large-cell lymphoma (ALCL). ALCL is a type of non-Hodgkin's lymphoma characterized by the expression of CD30/Ki-1 antigen. ALCL normally arises from T-cells, however, a subset of cases have either a null cell or B-cell phenotype. Cases that arise from B-cells are sometimes categorized as diffuse large B-cell lymphomas. About 60% of the ALCL case that express CD30/Ki-1 antigen also have the chromosomal translocation t(2; 5)(p23; q35) which fuses the nucleophosmin (NPM/B23) gene to the anaplastic lymphoma kinase (ALK) gene and results in an oncogenetic fusion protein NPM-ALK which has tyrosine kinase activity. Within specific subtypes of ALCL, ALK rearrangements have been observed in the following percentages: 1) 30% to 50% of pleomorphic ALCL, 2) more than 80% of monomorphic ALCL, 3) 75% to 100% of small-cell cases, and 4) 60% to 100% of lymphohistiocytic ALCL. NPM-ALK is able to transform both fibroblasts, hematopoietic, and primary bone marrow cell lines, and is thought to stimulate mitosis through the RAS pathway and the through activation of phospholipase C-gamma (PLC-gamma), and to protect against apoptosis through its activation of phosphatidylinositol 3 kinase (PI-3 kinase) survival pathway. (Duyster, et al., Oncogene (2001), 20:5623-5637). NPM-ALK has been shown to associate with Hsp90 and incubation of NPM-ALK expressing ALCL cells with the benzoquinone ansamycin, 17AAG, has been shown to disrupt this association resulting in increased degradation of NPM-ALK and induce cell-cycle arrest and apoptosis. (Georgakis, et al., Exp. Hematology (2006), 34(12):1670-1679; Bonvini, et al., Cancer Research (2002), 62:1559-1566).
Although promising, benzoquinone ansamycins, and their derivatives, suffer from a number of limitations. For example, they have low oral bioavailability, and their limited solubility makes them difficult to formula. In addition, they are metabolized by polymorphic cytochrome P450 CYP3A4 and are a substrate for P-glycoprotein export pump involved in the development of multidrug resistance. Therefore, a need exist for new therapeutics that improve the prognosis of cancer patients and that reduces or overcomes the limitations of currently used anti-cancer agents.